The endemic Ancylidae of Lake Ochrid (three species of Ancylus and two species of Acroloxus), have been described conchologically and anatomically. One of the species, Ancylus lapicidus, is new. Another of the species, Acroloxus macedonicus, occurs in separated populations with different morphological characteristics. These characteristics are concluded to be genotypical. Two species are confined to the shore zone, three to the shell zone. These two depth zones are separated by the intermediary Chara zone. The occurrence of the ancylid populations is spotted.
The distribution of the ancylid species in Lake Ochrid is discussed; the spotted distribution of their populations, and the occurrence of Ancylidae in the Palearctic region in general leads to the conclusion that intralacustrine speciation is probable among Ancylidae in Lake Ochrid.
It is pointed out that the term “spatial isolation” is much to be preferred to “geographical isolation” in confined areas and therefore would have a more universal applicability.